(1) Field of the Invention
The present invention relates to the field of interferometry.
(2) Description of Related Art
Interferometers are devices that measure the interference pattern produced by the superposition of two or more waves, such as those of electromagnetic radiation. Interferometers can be used, in particular, for the accurate measurement of changes in distance (resolution of up to 1 pm is possible in some cases, with a 1 second averaging time).
In the standard (Michelson) design of interferometer, a beam of light from a light source is split into two, by means of a beam splitter. One beam of light (the reference beam) is directed towards a reflector (which may be a plane mirror, or a retroreflector such as a cube corner) housed within the interferometer, whilst the other is directed towards a reflector on the target. Following reflection of the two beams from the respective reflectors, the beams of light are recombined, and the resulting interference measured by a detector. Since the reference reflector is held at a fixed position relative to the beamsplitter whilst the target reflector is separate from the interferometer, any movement of the target relative to the interferometer will change the interference pattern in a predictable manner, allowing changes in the relative distance between the interferometer and target to be calculated. In modern devices, the beam of light frequently has a high degree of coherence, such as a light beam from a laser device.
One modern development of a Michelson interferometer is described in Class. Quantum Gray., vol. 22 (2005), pages 1-9. Another such device is sold by Canon, Inc. under the DS-80 model name.
One problem with these prior art designs is that, in order to function correctly, the target reflector must be correctly aligned with the axis of the light beam, which generally means that the normal axis of the reflector should be coincident with the axis of the incident light beam. Thus, for example, in one embodiment, a misalignment of only 1 mrad of the target leads to a 2-fold reduction in fringe visibility. Although some divergence of these axes is possible for certain constructions, the tolerance for misalignment applies only over a narrow range of distances, and drops markedly if the distance from the interferometer to the target is outside this range. In the Canon model, the range over which the device can be used is about 50 μm.
Since the accuracy with which the target reflector can be oriented is frequently limited, the usefulness of any given interferometer is limited to measuring changes in distance within this narrow range. Thus, different configurations of interferometer are required for different applications, according to the distance to be measured. Furthermore, increasing the usable distance of the interferometer may also require other dimensions to be increased, such that the device becomes large and unwieldy.
It is possible to use a retroreflector as the target reflector, in which case the tolerance for misalignment of the reflector is increased. Retroreflectors are optical devices which are able to reflect a beam of light in a direction parallel with the incident beam, even where that incident beam is not axially aligned with the reflector. Common retroreflectors include half-silvered glass spheres, and corner-cube reflectors. However, it is not always practical to mount retroreflectors on the target. For example, where the target is very low in mass, the additional mass of the retroreflector may significantly alter the dynamic properties of the target. Where it is desired to measure distances of a large number of similar or identical components (e.g. on a production line) it may be impractical to apply retroreflectors to each individual component. Furthermore, although the light beam reflected from a retroreflector may be parallel with the incident beam, it will usually be laterally displaced from the incident beam axis, due to translation within the retroreflector. This reduces the degree of overlap and hence the visibility of interference fringes within the interferometer.
It is therefore desirable to have an interferometer for measuring distance with increased tolerance for axial misalignment.